GM's second-generation Voltec drive unit in the 2016 Chevolet Volt integrates within it a downsized DC-to-AC power inverter that uses conventional silicon semiconductors.

North
Carolina-based Cree, Inc. introduced the industry’s first silicon carbide 900V
MOSFET power electronics device in early 2015.

Power inverters convert DC to AC using semiconductor diodes and transistors to control electric traction motors.

Inside every
plug-in vehicle there’s a black box the size of a six-pack cooler that connects
the battery to the electric motor. It’s called the power inverter. This
crucial, but often overlooked component converts the battery pack’s high-voltage direct current (DC) into alternating current (AC) pulses that control the traction motor.

A DC-to-AC inverter,
basically a fast-acting silicon semiconductor switch, functions something like
an Engine Management System does in a internal-combustion power plant. It feeds the driver’s commands to the traction motor in the form of pulse-width-modulated drive signals at frequencies that can range from 10 Hz to 10 kHz.

Because
all electric traction power passes through the inverter, any efficiency losses
that occur within cut directly into a plug-in vehicle’s battery-only driving
range.

In fact, more efficient inverter technology ranks second in importance only
to more power-dense batteries for extending battery-only range in
next-generation plug-ins.

Improved
electric and hybrid vehicles are not alone in their need for better inverters.
High-efficiency inverter technology would also greatly benefit industrial motors,
consumer electronics, appliances and data centers as well as photovoltaic and
wind energy systems.

It’s no surprise
then that electronics and materials researchers worldwide are working to
develop improved semiconductors that could deliver inverter performance that is superior to conventional silicon—including fewer switching losses, greater thermal
efficiency and importantly, reduced system costs. Even Google is working on this
issue, having established last year a prize competition—The Little Box Challenge—that will award $1 million to
the developer of the best inverter design for "green energy" applications; see https://www.littleboxchallenge.com/.

The goal of this
research is to develop what are called wide bandgap (WBG) semiconductors. To physicists,
WBG materials exhibit a relatively large quantum energy range in which no
electron states can exist—a bigger electron energy gap compared to silicon between the top of
the valence band and the bottom of the conduction band. In practice, it refers to the amount of energy that is needed to release electrons from a particular semiconductor
material for conduction.

Semiconductors
with wider bandgaps can, for example, withstand higher applied electric fields,
or voltages, as well as operate at higher temperatures, power densities
and frequencies.

In the
automotive sector, the U.S. Department of Energy recently awarded research grants
to General Motors ($3.99 million) and Delphi ($2.46 million) to support three-year,
cost-shared projects to develop high-efficiency, cost-competitive
integrated power inverter modules based on WGB semiconductors for plug-in
vehicles.

Automotive Engineering previously reported on Toyota’s continuing research efforts to develop more efficient automotive power electronics modules using silicon carbide; see http://articles.sae.org/13244/.

Smaller, lighter
inverters

Inverters in current
plug-ins rely on silicon-based power transistor technology that was developed for
industrial applications over the last 25 years, said Pete Savagian, GM's General
Director for Electric Drives and Systems Engineering and a veteran of the
company’s pioneering EV-1 program. These insulated-gate
bipolar transistors (IGBTs), often tuned for automotive use, combine good
efficiency and fast switching, he explained, but expanding plug-ins’ battery-only
driving range means moving beyond silicon.

Two emerging WBG
semiconductors, silicon carbide and gallium nitride, are expected to fill that
role, Savagian continued, because they “can bring three to ten times better energy
efficiency when they're turned on and especially, when they're turned off. And when you’re
switching at rates of 10,000 Hz, reducing losses becomes important.”

Wide-bandgap
inverter technology "plays upon the ability of the transistor material to run at higher
temperature and with fewer losses than silicon-based power electronics," explained A.J.
Lasley, Director of Electronic Controls Advanced Engineering at Delphi in
Indianapolis. “The improved efficiency can directly translate into longer range.”

He noted that wide-bandgap
materials, particularly silicon carbide semiconductors, have been trying to
push into industry for many years, with the recent DoE-supported
projects aiming to "push the limit" in plug-in inverter technology.

"The new
materials offer great potential for allowing us to reduce the size of inverters
by as much as 30% and cut energy losses by 20% to 30%,” Lasley said.

According to GM's Savagian, the new WBG semiconductors would allow “using less semiconductor material in
inverters than we do now. The resulting smaller footprint means that everything
else can shrink as well, including all the support equipment—electrical
connectors, cooling system, heat exchanger, and the housing and chassis
structures.”

Such
physical and operational downsizing should in addition yield significantly
cheaper power inverter units, Savagian predicted. He noted that the inverter typically
accounts for about 40% of the total cost of an electric drive train, which
includes an electric motor and a gear reduction system.

Both Savagian
and Lasley stressed that one of the principal benefits of WBG semiconductors to
plug-in vehicles is that they would enable engineers to integrate power
inverters directly into the transmission systems.

“Their smaller size means
that the mounting and packaging can be more rigid and robust,"
Savagian observed. "It also would enable engineers to incorporate the devices into the transmission units, saving space and weight. You could, for instance, get rid of the electrical cables, which
makes assembly easier.”

Beyond silicon

Experts note that gallium nitride
has similar bandgap characteristics to silicon carbide, which is a more mature technology. But silicon carbide chip fabrication "is very expensive, while
gallium nitride offers the possibility of lower-cost manufacturing because of
it is more compatible with the underlying substrate materials,” said Jayant Baliga, Director of the Power Semiconductor
Research Center at North Carolina State University. Baliga, a pioneer in power
electronics, invented and commercialized IGBT devices when he worked at General
Electric.

Baliga’s NCSU
center is taking the lead in the Power America program, also known as the Next Generation Power
Electronics National Manufacturing Innovation Institute. This is a five-year,
$140-million R&D effort established in January 2015 by the DoE “to drive WBG semiconductor costs to make them more competitive with silicon
materials.” In the case of silicon carbide, the researchers are attempting to adapt
existing silicon chip foundries to silicon carbide chip fabrication, Baliga noted.

Anant Agarwal, the senior WBG
expert at the DoE’s Advanced Manufacturing Office, has said he
expects that highly efficient power electronic devices using the new
semiconductors will be able to achieve price parity with traditional
silicon-based devices within about five years.

Power America’s members
comprise a dozen companies as well as seven universities and laboratories,
including ABB,Arkansas Power Electronics International,Cree, Delphi, John
Deere,Monolith Semiconductor,Qorvo,Toshiba,Transphorm,United Silicon
Carbide,VACON and X-FAB.

Besides NCSU, the program’s academic and lab partners
are Arizona State University, Florida State University, the National Renewable
Energy Laboratory, the U.S. Naval Research Laboratory, the University of
California, Santa Barbara and Virginia Polytechnic Institute and State
University.

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